Background: We examined the relationship of musculoskeletal risk factors underlying force and repetition on tissue responses in an operant rat model of repetitive reaching and pulling, and if force x repetition interactions were present, indicative of a fatigue failure process. We examined exposure-dependent changes in biochemical, morphological and sensorimotor responses occurring with repeated performance of a handle-pulling task for 12 weeks at one of four repetition and force levels: 1) low repetition with low force, 2) high repetition with low force, 3) low repetition with high force, and 4) high repetition with high force (HRHF).

Methods: Rats underwent initial training for 4-6 weeks, and then performed one of the tasks for 12 weeks, 2 hours/day, 3 days/week. Reflexive grip strength and sensitivity to touch were assayed as functional outcomes. Flexor digitorum muscles and tendons, forelimb bones, and serum were assayed using ELISA for indicators of inflammation, tissue stress and repair, and bone turnover. Histomorphometry was used to assay macrophage infiltration of tissues, spinal cord substance P changes, and tissue adaptative or degradative changes. MicroCT was used to assay bones for changes in bone quality.

Conclusions: Although not consistent in all tissues, we found several significant interactions between the critical musculoskeletal risk factors of force and repetition, consistent with a fatigue failure process in musculoskeletal tissues. Prolonged performance of HRHF tasks exhibited significantly increased risk for musculoskeletal disorders, while performance of moderate level tasks exhibited adaptation to task demands.

Figure 5: Peripheral and central neural responses examined using quantitative immunohistochemical methods in week 0 (immediately following the training period) or after performing either a LRLF, HRLF, LRHF and HRHF task for 12 weeks. (A & B) Mean number of ED1-immunoreactive (ED1-IH) macrophages in the median nerve at the level of the wrist, in week 0 and 12. (C & D) Representative photos of the median nerve at the level of the wrist, showing an absence of ED1-IH macrophages in a NC rat, but increased macrophages (stained black; arrows indicate a few) in a 12-wk HRHF rat. (E & F) Percent area with substance P immunoreactivity in the dorsal horns of lower cervical spinal cord segments, in week 0 and 12. Data for upper lamina (I and II) of the dorsal horns are presented. (G & H) Representative photos of the dorsal horn of lower cervical spinal cord segments, showing only low grade increases of substance P (SubP) immunoreactivity in a NC rat, but increased SubP in the upper lamina (arrows) in a 12-wk HRHF rat. Symbols: aa: p < 0.01, compared to LRLF rats; bb: p < 0.01, compared to HRLF rats; **: p < 0.01, compared to normal controls (NC) rats (indicated by dashed line). Mean and SEM are shown. Scale bars are as indicated.

Mentions:
Neuritis, in the form of increased activated macrophages in the median nerve at the wrist, was affected mainly by force at 12-weeks of task performance (Figure 5B). Specifically, increased macrophages were observed in 0-week HRHF, 12-week HRLF, 12-week LRHF rats, and 12-week HRHF, compared to NC rats (Figure 5A,B), and in 12-week LRHF and HRHF rats, compared to 12-week LRLF rats (Figure 5B). Figure 5D shows an increase of ED1-immunoreactive (ED1-IH; activated) macrophages in intraneural and extraneural regions of the median nerve at the level of the wrist in 12-week HRHF rats (arrows indicate representative macrophages), compared to an absence in NC rats (Figure 5C).

Figure 5: Peripheral and central neural responses examined using quantitative immunohistochemical methods in week 0 (immediately following the training period) or after performing either a LRLF, HRLF, LRHF and HRHF task for 12 weeks. (A & B) Mean number of ED1-immunoreactive (ED1-IH) macrophages in the median nerve at the level of the wrist, in week 0 and 12. (C & D) Representative photos of the median nerve at the level of the wrist, showing an absence of ED1-IH macrophages in a NC rat, but increased macrophages (stained black; arrows indicate a few) in a 12-wk HRHF rat. (E & F) Percent area with substance P immunoreactivity in the dorsal horns of lower cervical spinal cord segments, in week 0 and 12. Data for upper lamina (I and II) of the dorsal horns are presented. (G & H) Representative photos of the dorsal horn of lower cervical spinal cord segments, showing only low grade increases of substance P (SubP) immunoreactivity in a NC rat, but increased SubP in the upper lamina (arrows) in a 12-wk HRHF rat. Symbols: aa: p < 0.01, compared to LRLF rats; bb: p < 0.01, compared to HRLF rats; **: p < 0.01, compared to normal controls (NC) rats (indicated by dashed line). Mean and SEM are shown. Scale bars are as indicated.

Mentions:
Neuritis, in the form of increased activated macrophages in the median nerve at the wrist, was affected mainly by force at 12-weeks of task performance (Figure 5B). Specifically, increased macrophages were observed in 0-week HRHF, 12-week HRLF, 12-week LRHF rats, and 12-week HRHF, compared to NC rats (Figure 5A,B), and in 12-week LRHF and HRHF rats, compared to 12-week LRLF rats (Figure 5B). Figure 5D shows an increase of ED1-immunoreactive (ED1-IH; activated) macrophages in intraneural and extraneural regions of the median nerve at the level of the wrist in 12-week HRHF rats (arrows indicate representative macrophages), compared to an absence in NC rats (Figure 5C).

Background: We examined the relationship of musculoskeletal risk factors underlying force and repetition on tissue responses in an operant rat model of repetitive reaching and pulling, and if force x repetition interactions were present, indicative of a fatigue failure process. We examined exposure-dependent changes in biochemical, morphological and sensorimotor responses occurring with repeated performance of a handle-pulling task for 12 weeks at one of four repetition and force levels: 1) low repetition with low force, 2) high repetition with low force, 3) low repetition with high force, and 4) high repetition with high force (HRHF).

Methods: Rats underwent initial training for 4-6 weeks, and then performed one of the tasks for 12 weeks, 2 hours/day, 3 days/week. Reflexive grip strength and sensitivity to touch were assayed as functional outcomes. Flexor digitorum muscles and tendons, forelimb bones, and serum were assayed using ELISA for indicators of inflammation, tissue stress and repair, and bone turnover. Histomorphometry was used to assay macrophage infiltration of tissues, spinal cord substance P changes, and tissue adaptative or degradative changes. MicroCT was used to assay bones for changes in bone quality.

Conclusions: Although not consistent in all tissues, we found several significant interactions between the critical musculoskeletal risk factors of force and repetition, consistent with a fatigue failure process in musculoskeletal tissues. Prolonged performance of HRHF tasks exhibited significantly increased risk for musculoskeletal disorders, while performance of moderate level tasks exhibited adaptation to task demands.